Comparison of 12 Quadrature Birdcage Coils with Different Leg Shapes at 9.4 T

The purpose of this study was to analyse the relationship between the radio frequency (RF) coil performance and conductor surface shape for ultra-high field (UHF) magnetic resonance imaging. Twelve different leg-shaped quadrature birdcage coils were modeled and built, e.g., 4 mm-width-leg conventional birdcage coil, 7 mm-width-leg conventional birdcage, 10 mm-width-leg conventional birdcage coil, 13 mm-width-leg conventional birdcage coil, inside arc-shape-leg birdcage coil, outward arc-shape-leg birdcage coil, inside right angle-shape-leg birdcage coil, outward right angle-shape-leg birdcage coil, vertical 4 mm-width-leg vertical birdcage, 6 mm-width-leg vertical birdcage, 8 mm-width-leg vertical birdcage and 10 mm-width-leg vertical birdcage. Studies were carried out in both electromagnetic simulations with finite element method as well as in vitro saline phantom experiments at 9.4 T. Both the results of simulation and experiment showed that conventional birdcage coil produces the highest signal-to-noise ratio (SNR) while the vertical birdcage coil produces the most homogeneous RF magnetic (B ₁) field at UHF. For conventional birdcage coils, as well as the vertical birdcage coils, only the proper width of legs results in the best performance (e.g., B ₁ homogeneous and SNR). For vertical birdcage coils, the wider the leg size, the higher RF magnetic (B ₁) field intensity distribution.     

A volume microstrip RF coil for MRI microscopy

Quantitative magnetic resonance imaging (MRI) studies of small samples such as a single cell or cell clusters require application of radiofrequency (RF) coils that provide homogenous B₁ field distribution and high signal-to-noise ratio (SNR).We present a novel design of an MRI RF volume microcoil based on a microstrip structure. The coil consists of two parallel microstrip elements conducting RF currents in the opposite directions, thus creating homogenous RF field within the space between the microstrips. The construction of the microcoil is simple, efficient and cost-effective.Theoretical calculations and finite element method simulations were used to optimize the coil geometry to achieve optimal B₁ and SNR distributions within the sample and predict parameters of the coil. The theoretical calculations were confirmed with MR images of a 1-mm-diameter capillary and a plant obtained with the double microstrip RF microcoil at 11.7 T. The in-plane resolution of MR images was 24 μm×24 μm.     

An optimized solenoidal head radiofrequency coil for low-field magnetic resonance imaging

Applications of low-field magnetic resonance imaging (MRI) systems (<0.3 T) are limited due to the signal-to-noise ratio (SNR) being lower than that provided by systems based on superconductive magnets (≥1.5 T). Therefore, the design of radiofrequency (RF) coils for low-field MRI requires careful consideration as significant gains in SNR can be achieved with the proper design of the RF coil. This article describes an analytical method for the optimization of solenoidal coils. Coil and sample losses are analyzed to provide maximum SNR and optimum B₁ field homogeneity. The calculations are performed for solenoidal coils optimized for the human head at 0.2 T, but the method could also be applied to any solenoidal coil for imaging other anatomical regions at low field. Several coils were constructed to compare experimental and theoretical results. A head magnetic resonance image obtained at 0.2 T with the optimum design is presented.     

Design and Analysis of Microstrip-Based RF Birdcage Coil for 1.5 T Magnetic Resonance Imaging

Magnetic resonance imaging (MRI) technique is widely used to capture the images of the liquid items inside the human body. The radio-frequency (RF) coil is one of the important modules present inside an MRI system, which plays a major role in image quality. In this work, a microstrip-based high-pass RF birdcage coil is proposed for 1.5 T MRI. The cylindrical-shaped birdcage coil consists of 12 microstrip radiating elements and tuning capacitors to achieve a resonance at 63.85 MHz. The coil is made up of 10 mm polytetrafluoroethylene substrate coated by a conducting transmission line of desired length and width. A finite difference time domain simulation is carried out to analyze the return loss (S₁₁), magnetic field homogeneity and Specific Absorption Rate (SAR) parameters of the RF coil. The SAR values of the proposed microstrip-based 1.5 T birdcage coil was compared with 3 T RF birdcage coil. The simulation results indicate the proposed birdcage coil structure gives optimal values of S₁₁, magnetic field homogeneity and SAR.     

Design considerations and coil comparisons for 7 T brain imaging

The development of 300 MHz radio-frequency (RF) head coils analogous to those used at field strengths of 1.5 and 3 T is complicated by increased dissipative losses in conductive tissue, effects arising from the short RF wavelength in biological tissue (about 13 cm at 300 MHz), and the constraints imposed by the use of head gradient sets desirable for mitigating increased static field susceptibility effects. In this study, five RF head coils were constructed and tested on a 7 T scanner including 2 TEM designs, 2 birdcage designs and a local receive-only array. Signal-to-noise ratio, coil reception profiles and interactions between the coil and dielectric head were examined. Particular attention was placed on the coil’s reception in the neck and shoulders, where the head gradient is unable to spatially encode the image. With the use of conductive shields and distributed capacitance, all of the coil designs could be made to image effectively at high field, but each design was found to have subtle differences in field distribution, interaction with the dielectric boundary conditions of the head and fringe fields in the neck and shoulders. In particular, the birdcage and array coils were found to have reducedB ₁ reception field profiles in the neck and shoulders which helped reduce signal detection outside the linear region of the head gradient coil. Although the TEM coils exhibited higher signal detection in the neck and shoulders, all the coils picked up enough signal from these regions to produce artifacts in the brain. These artifacts could be mitigated through use of a conductive shield or by small local dephasing shims sewn into the shoulders of a jacket worn by the subject. Although homogeneous in low-dielectric-constant phantoms, the volume coil’sB ₁ profile was strongly peaked in the center of the head, rendering them spatially complementary to that observed in the surface coil array. The image profile of the surface coil was found to be less dramatically changed from patterns observed at lower field strength. Its dielectric brightening pattern was found to depend on the orientation of the coil with respect to the head.     

低场永磁体磁共振射频场映像

射频场映像是通过一定算法对磁共振射频线圈的发射场进行重建的方法.高场下的射频场经过生物组织时会发生明显变化,在其基础上可以反演生物组织体内电特性,进而对癌症等疾病进行早期诊断,是对生物组织的磁共振结构成像的有力补充.目前为止,射频场映像和电特性研究都以高场鸟笼线圈为主,对低场下的相控阵研究较少.本文主要研究了低场永磁体磁共振射频场的均匀度.有限元仿真和实验验证了在17.8 MHz激励下,射频场在空载和负载下均匀度均发生较大变化.射频场均匀度在负载下的改变在一定程度上可以反映负载生物组织的电特性,对磁共振电特性实用化研究提供了一定的参考价值.     

一种可拆装式的脑外科手术用磁共振成像线圈

在临床磁共振成像(MRI)应用中,射频线圈的设计是非常关键的,针对不同的应用目的,合适的线圈能获得质量更好的图像. 有的应用需要线圈提供均匀性较好的射频场,而有的应用则需要线圈在特定区域内提供高的信噪比(SNR). 但是线圈很难同时得到好的射频场（B₁场）、空间均匀性和高的SNR,需要根据实际应用情况进行折衷设计. 针对MRI在脑外科手术中的应用特点,设计并制作了一种新颖的、适用于脑外科手术的MRI接收和发射共用射频线圈. 该线圈采用可分拆式结构,在脑外科手术支架上可以进行反复组装和拆卸,减少了MRI对医生手术的影响. 仿真结果和人体成像实验表明,该线圈能产生均匀的射频场、有较高的SNR和较大的成像范围,满足脑外科手术的需要.     

Advanced theory of driven birdcage resonator with losses for biomedical magnetic resonance imaging and spectroscopy

A complete time-dependent physics theory of symmetric unperturbed driven hybrid birdcage resonator was developed for general application. In particular, the theory can be applied for radiofrequency (RF) coil engineering, computer simulations of coil-sample interaction, etc. Explicit time dependence is evaluated for different forms of driving voltage. The major steps of the solution development are shown and appropriate explanations are given. Green's functions and spectral density formula were developed for any form of periodic driving voltage. The concept of distributed power losses based on transmission line theory is developed for evaluation of local losses of a coil. Three major types of power losses are estimated as equivalent series resistances in the circuit of the birdcage resonator. Values of generated resistances in legs and end-rings are estimated. An application of the theory is shown for many practical cases. Experimental curve of B₁ field polarization dependence is measured for eight-sections birdcage coil. It was shown that the steady-state driven resonance frequencies do not depend on damping factor unlike the free oscillation (transient) frequencies. An equivalent active resistance is generated due to interaction of RF electromagnetic field with a sample. Resistance of the conductor (enhanced by skin effect), Eddy currents and dielectric losses are the major types of losses which contribute to the values of generated resistances. A biomedical sample for magnetic resonance imaging and spectroscopy is the source of the both Eddy current and dielectric losses of a coil. As demonstrated by the theory, Eddy current loss is the major effect of coil shielding.     

Optimization of a quadrature birdcage coil for functional imaging of squirrel monkey brain at 9.4T

A quadrature transmit/receive birdcage coil was optimized for squirrel monkey functional imaging at the high field of 9.4 T. The coil length was chosen to gain maximum coil efficiency/signal-to-noise ratio (SNR) and meanwhile provide enough homogenous RF field in the whole brain area. Based on the numerical simulation results, a 16-rung high-pass birdcage coil with the optimal length of 9 cm was constructed and evaluated on phantom and in vivo experiments. Compared to a general-purpose non-optimized coil, it exhibits approximately 25% in vivo SNR improvement. In addition to the volume coil, details about how to design and construct the associated animal preparation system were provided.     

Intracranial microvascular imaging at 7 T MRI with transceiver RF coils

Purpose

To investigate intracranial microvascular images with transceiver radio-frequency (RF) coils at ultra-high field 7 T magnetic resonance imaging (MRI).

Materials and methods

We designed several types of RF coils for the study of 7 T magnetic resonance angiography and analyzed quantitatively each coil's performance in terms of the signal-to-noise ratio (SNR) profiles to evaluate the usefulness of RF coils for microvascular imaging applications. We also obtained the microvascular images with different resolutions and parallel imaging technique.

Results

The overlapped 6-channel (ch) transceiver coil exhibited the highest performance for angiographic imaging. Although other multi-channel coils, such as 4- or 8-ch, were also suitable for fast imaging, these coils performed poorly in homogeneity or SNR for angiographic imaging. Furthermore, the 8-ch coil was poor in SNR at the center of the brain, while it had the highest SNR at the periphery.

Conclusion

The present study has demonstrated that the overlapped 6-ch coil with large-size loop coils provided the best performance for microvascular imaging or angiography with the ultra-high-field 7 T MRI, mainly because of its long penetration depth together with high SNR.     

Brain Imaging with Slotted Hybridized Magnetic Metamaterial Hat at 7-T MRI

Study of human pathologies and acquisition of anatomical images without any surgical intervention inside human body is possible because of magnetic resonance imaging (MRI), which is the keystone technique to characterize the psychology and neurochemistry of human body. However, for clinical trials, the study and cure of human diseases are followed by medical investigations of different animal anatomies. By employing different imaging techniques to animal anatomical models during their clinical trials yielded in exceptional image acquisition without any surgical invasion in the model, which resulted in noninvasive technique as compared to surgical invasion and opened the possibility to study human pathologies more precisely. This work exploits the notable properties of unique combination of multi-circular hybridized surface coils which can be used as hybridized magnetic metamaterial hat (HMMH). HMMH not only strengthens the uniformity of radio frequency (RF) rotational symmetry around its axis but also improves the signal-to-noise ratio (SNR) for rat’s brain imaging at 7-T MRI. We analyzed a periodic array of strongly coupled circular copper coils attached on circular coil shaped printed circuit board (PCB) substrate. In the design, some copper coils were inspired by the slot cavity loaded with parametric elements (capacitor and inductor). In addition, coils in the form of HMMH exploited the advantages of the hybrid modes which exhibited better and deeper RF sensitivity into the region of interest (ROI) as compared to single loop RF coil by exciting two Eigen modes simultaneously which resulted in homogenized magnetic field (B-field) and enhanced SNR at ROI. At resonance, the value of relative negative permeability, μ _r  = ?7 + j11 was achieved at 300 MHz for 7-T MRI. Furthermore, image quality at ROI was optimized by varying rat’s head position under magnetic resonance (MR) coil of MRI apparatus and in the presence or absence of HMMH. Design configuration and circuit model analysis were also done.     

The effects of coupled B1 fields in B1 encoded TRASE MRI - A simulation study

Transmit Array Spatial Encoding (TRASE) is a novel MRI technique that encodes spatial information by introducing phase gradients in the transmit RF (B₁) magnetic field. Since TRASE relies on the use of multiple RF fields (B₁ fields with different phase gradients) for k-space traversal, a TRASE pulse sequence requires RF pulses that are produced by switching between the transmit coils (B₁ fields). However, interactions among the transmit RF coils can cause un-driven coils to produce unwanted B₁ fields that impair the spatial encoding. Therefore, TRASE is sensitive to B₁ field perturbations arising from inductive coupling among the RF transmit coils and any B₁ field isolation (coil decoupling) technique requires an understanding of the effects of the B₁ field interactions. The purpose of this study was to investigate the effects of B₁ field coupling using Bloch equation based simulations and to determine the acceptable level of B₁ field interactions for 2D TRASE imaging. The simulations show that 2D TRASE MRI (using a 3-coil setup) displays ideal performance for pairwise coupling constant lower than k = 0.01 while having acceptable performance up to k = 0.1. This translates into S₁₂ measurements of range ~(− 50 dB to −30 dB) required for successful 2D TRASE MRI in this study. This result is of crucial importance for designers of practical TRASE transmit array systems.     

A Degenerate Birdcage with Integrated Tx/Rx Switches and Butler Matrix for the Human Limbs at 7 T

The theoretically known degeneracy condition of the band-pass birdcage coil has rarely been exploited in transmit coil designs. We have created an eight-channel degenerate birdcage for the human limbs at 7 T, with dedicated Tx/Rx switches and a Butler matrix. The coil can be split into two half cylinders, as required for its application to patients with limited mobility. The design of the coil, the Butler matrix, and Tx/Rx switches relied on a combination of analytical, circuital, and numerical simulations. The birdcage theory was extended to the degenerate case. The theoretical and practical aspects of the design and construction of the coil are presented. The performance of the coil was demonstrated by simulations, workbench, and scanner measurements. The fully assembled prototype presents good performance in terms of efficiency, B₁ homogeneity, and signal-to-noise ratio, despite the asymmetry introduced by the splittable design. The first in vivo images of the knee are also shown. A novel RF coil design consisting of an eight-channel splittable degenerate birdcage has been developed, and it is now available for 7 T MRI applications of the human lower limbs, including high-resolution imaging of the knee cartilages and of the patellar trabecular structure.     

An Efficacious Target-Field Approach to Design Shim Coils for Halbach Magnet of Mobile NMR Sensors

The main magnetic fields of mobile nuclear magnetic resonance (NMR) magnets differ from those of conventional NMR and magnetic resonance imaging (MRI) magnets. In the Halbach magnet, the main field B ₀ is perpendicular to the longitudinal axis, the symmetry of the current distribution with respect to the symmetry of the magnetic field differs from that in conventional target-field applications, and the current distribution on the coil surface cannot be expressed in terms of periodic basis functions. To obtain the winding pattern of the coil, an efficacious target-field approach. The surface of a coil is divided into small discrete elements, where each element is represented by a magnetic dipole. From the stream function of the elements, the resultant magnetic field is calculated. The optimization strategy follows an objective function defined by the power dissipation or efficiency of the coil. This leads to the optimum stream function on the coil surface, whose contour lines define the winding patterns of the coil. This paper shows winding patterns designed of shim coils for Halbach magnet and illustrates the craft of a shim coil using flexible printed circuit board. The performance of the coils is verified by simulating the fields they produce over the sensitive volume.     

Design and Demonstration of Four-Channel Received Coil Arrays for Vertical-Field MRI

The layout of radio-frequency received coils is related to signal-to-noise ratio (SNR) in magnetic resonance imaging (MRI). In this paper, different structures of four-channel received coil arrays for vertical-field MRI are constructed and optimized by establishing the relationship between coil geometry and SNR to achieve a high SNR and a uniform SNR distribution in the region of interest (ROI). Then, the SNR distributions of three optimized configurations, including rectangular loops, non-definite shape surface coils, and solenoid loops as the main unit, are simulated and compared. The four-channel coil of solenoid loops as the main unit has been found to have the best performance with the highest mean SNR in the ROI when imaging without acceleration. In addition, g-factor and 2D SENSE SNR in yoz-plane are simply analyzed, which show a sharp decrease in SNR for all the coils. Finally, all the coils are manufactured and operated at a 0.5 T permanent magnet MRI system with phantom and joint imaging experiments. Using pixel-by-pixel manner to evaluate SNR map, the experimental results are consistent with the simulation results, while parallel imaging experiment results show that the major consideration in low field MRI is the improvement of SNR value and uniformity rather than that of the imaging speed. As different constructions of four-channel received coils are investigated, we have found the most effective configuration with high and uniform SNR for vertical-field MRI.     

Multichannel transceiver dual-tuned RF coil for proton/sodium MR imaging of knee cartilage at 3 T

Sodium magnetic resonance (MR) imaging is a promising technique for detecting changes of proteoglycan (PG) content in cartilage associated with knee osteoarthritis. Despite its potential clinical benefit, sodium MR imaging in vivo is challenging because of intrinsically low sodium concentration and low MR signal sensitivity. Some of the challenges in sodium MR imaging may be eliminated by the use of a high-sensitivity radiofrequency (RF) coil, specifically, a dual-tuned (DT) proton/sodium RF coil which facilitates the co-registration of sodium and proton MR images and the evaluation of both physiochemical and structural properties of knee cartilage. Nevertheless, implementation of a DT proton/sodium RF coil is technically difficult because of the coupling effect between the coil elements (particularly at high field) and the required compact design with improved coil sensitivity. In this study, we applied a multitransceiver RF coil design to develop a DT proton/sodium coil for knee cartilage imaging at 3 T. With the new design, the size of the coil was minimized, and a high signal-to-noise ratio (SNR) was achieved. DT coil exhibited high levels of reflection S11 (～-21 dB) and transmission coefficient S12 (～-19 dB) for both the proton and sodium coils. High SNR (range 27-38) and contrast-to-noise ratio (CNR) (range 15-21) were achieved in sodium MR imaging of knee cartilage in vivo at 3-mm(3) isotropic resolution. This DT coil performance was comparable to that measured using a sodium-only birdcage coil (SNR of 28 and CNR of 20). Clinical evaluation of the DT coil on four normal subjects demonstrated a consistent acquisition of high-resolution proton images and measurement of relative sodium concentrations of knee cartilages without repositioning of the subjects during the same MR scanning session.     

Hyperpolarized 13C MRS Cardiac Metabolism Studies in Pigs: Comparison Between Surface and Volume Radiofrequency Coils

Cardiac metabolism assessment with hyperpolarized ¹³C magnetic resonance spectroscopy in pig models requires the design of dedicated coils capable of providing large field of view with high signal-to-noise ratio (SNR) data. This work presents a comparison between a commercial ¹³C quadrature birdcage coil and a homebuilt ¹³C circular coil both designed for hyperpolarized studies of pig heart with a clinical 3T scanner. In particular, the simulation of the two coils is described by developing an SNR model for coil performance prediction and comparison. While coil resistances were calculated from Ohm’s law, the magnetic field patterns and sample-induced resistances were calculated using a numerical finite-difference time-domain algorithm. After the numerical simulation of both coils, the results are presented as SNR-versus-depth profiles using experimental SNR extracted from the [1-¹³C]acetate phantom chemical shift image and with a comparison of metabolic maps acquired by hyperpolarized [1-¹³C]pyruvate injected in a pig. The accuracy of the developed SNR models was demonstrated by good agreement between the theoretical and experimental coil SNR-versus-depth profiles.     

Composite pulses for RF phase encoded MRI: A simulation study

BackgroundIn B₁ encoded MRI, a realistic non-linear phase RF encoding coil will generate an inhomogeneous B₁ field that leads to spatially dependent flip angles. The non-linearity of the B₁ phase gradient can be compensated for in the reconstruction, but B₁ inhomogeneity remains a problem. The effect of B₁ inhomogeneity on tip angles for conventional, B₀ encoded MRI, may be minimized using composite pulses. The objective of this study was to explore the feasibility of using composite pulses with non-linear RF phase encoding coils and to identify the most appropriate composite pulse scheme.MethodsRF encoded signals were simulated via the Bloch equation for various symmetric, asymmetric and antisymmetric composite pulses. The simulated signals were reconstructed using a constrained least squares method.ResultsRoot mean square reconstruction errors varied from 6% (for an asymmetric composite pulse) to 9.7% (for an antisymmetric composite pulse).ConclusionAn asymmetric composite pulse scheme created images with fewer artifacts than other composite pulse schemes in inhomogeneous B₀ and B₁ fields making it the best choice for decreasing the effects of spatially varying flip angles. This is contrary to the conclusion that antisymmetric composite pulses are the best ones to use for spin echo sequences in conventional, B₀ encoded, MRI.     

4T split TEM volume head and knee coils for improved sensitivity and patient accessibility

Split RF coils offer improved patient access by eliminating the need for the coil to be slid over the region of interest. For unshielded birdcage coils, the presence of end ring currents necessitates a direct electrical connection between two halves of the coil. For high-field (>3T) shielded birdcage coils, both the shield and the coil must be split and reliably connected electrically. This problem can be circumvented by the use of split TEM volume coils. Since the elements of a TEM coil are coupled inductively, no direct electrical connection between the halves is necessary. In this work we demonstrate that the effects of splitting the shield for head and knee TEMs can be compensated for, and performance retained. For the knee, the improved access allowed the coil diameter to be reduced, enhancing the sensitivity by 15-20%.     

A Method for Accurate Calculation ofB1Fields in Three Dimensions. Effects of Shield Geometry on Field Strength and Homogeneity in the Birdcage Coil

A method for calculatingB₁field strength and homogeneity as functions of radiofrequency shield geometry is presented. The method requires use of three-dimensional finite-element analysis, birdcage-coil theory, and antenna-array theory. Calculations were performed for a 12-element birdcage coil (19 cm diameter, 21 cm length) at 125 MHz. CalculatedB₁field strengths and homogeneities for the coil in 25 different shields and in no shield are given. For configurations where the shield is longer than the coil, bothB₁field strength and homogeneity decrease as shield diameter decreases or as shield length increases. In configurations where the shield is shorter than the coil and has a diameter of 25.6 cm,B₁homogeneity is greater than in an unshielded coil.B₁field strength was measured experimentally at 125 MHz in a birdcage coil of the same geometry as the model within shields of four different diameters. Calculated results very closely matched experimental measurement.